Numerous industrial applications require the use of a flow of very clean air, such as clean rooms for microelectronics manufacture, various facilities in the nuclear power industry and driers for webs of indeterminate length like freshly coated photographic film. Typically, such clean air is provided by filtering with high efficiency particulate air filters, commonly referred to as HEPA filters, which are throw-away dry type filters having a minimum particle removal efficiency typically not less than 99.97 to 99.99 percent for 0.3 micron particles and a maximum clean flow resistance typically not more than 0.5 to 1.0 inch of water at rated flow capacity. Such filters come in a variety of face dimensions and depths and typically have design flow capacities, at maximum flow resistance, from 25 to 1000 standard cubic feet per minute. In web drier installations familiar to the Applicants, such filters are positioned with the plane of the filter substantially vertical in arrays or banks housed in an enclosure remote from the point at which the clean air is needed. This known arrangement has been considered desirable to facilitate inspection and replacement of the filters. Air to be cleaned is forced through the filters by a suitable fan and the clean air is then brought to the point of use through ducts, which typically are made from stainless steel to minimize potential for contamination of the clean air with oxide particles from the duct walls and the like.
A number of disadvantages result from this known arrangement. The remote filter banks and their enclosures are costly and consume a considerable volume of building space. The ducts between the filters and the point of use must be stainless steel to minimize contamination, which increases the cost of the ducting by as much as 50% compared, for example, to galvannealed steel. In spite of the high efficiency of the HEPA filters, the ducts downstream of them must be cleaned and inspected periodically to remove small amounts of contaminants generated within or aspirated into the ducts. Even when such periodic cleaning is done, particle contamination can still be unacceptably high, leading in the case of a drier to contamination of the web during drying. Where photographic film is being dried, this can require large amounts of expensive product to be discarded due to contamination. Thus, a need has existed for an improved method and apparatus for filtering air in which contamination from the duct work is reduced and which is simpler and less expensive than prior art methods and apparatus.